Polymerizable mesogenic compounds, which are also known as reactive mesogenic compounds, have been described in prior art for various purposes. For example, they can be aligned in their liquid crystal phase and subsequently polymerized in situ to give linear or crosslinked liquid crystal polymer films with uniform orientation of high quality. These films can be used for example as optical elements, like polarization filters as described in EP 0 397 263, broadband circular polarizers as described in EP 0 606 940 and WO 97/35219, or compensation or retardation films as described in WO 98/00475, WO 98/04651 or WO 98/12584.
For many applications it is preferred to use crosslinked polymer films, as these show higher stability and lower temperature dependence of the optical properties, compared to films of linear polymers.
For this purpose, often polymerizable compositions are used which typically comprise a mixture of direactive and monoreactive polymerizable mesogenic compounds.
The terms reactive or polymerizable mesogenic compound as used in the foregoing and the following comprise compounds with a rod-shaped, lath-shaped or disk-shaped mesogenic group, i.e. a group with the ability to induce mesophase behaviour. These compounds do not necessarily have to exhibit mesophase behaviour by themselves. It is also possible that these compounds show mesophase behaviour only in mixtures with other compounds or when the polymerizable mesogenic compounds or the mixtures comprising them are polymerized.
A di- or multireactive compound in this connection means a compound having a mesogenic group and two or more polymerizable terminal groups attached to one or both sides of the mesogenic group, either directly or via a spacer group. A monoreactive compound means a compound having a mesogenic group, to one side of which is attached a polymerizable terminal group, either directly or via a spacer, and to the other side of which is attached a non-polymerizable terminal group, like e.g. a polar group such as halogen or cyano or a chiral or achiral alkyl or alkoxy group.
A typical example of a direactive polymerizable mesogenic compound of the prior art is compound (A)
wherein R is H or CH3, which is described in EP 0 331 233 and in D. J. Broer et al., Makromol. Chem. 190, 2255-2268 (1989).
Typical examples of monoreactive polymerizable mesogenic compounds of the prior art are compounds (B) and (C)
wherein R is e.g. CN or OCH3, which is described in DE 27 22 589,
which is described in DE 195,04,224.